This invention relates to a new improved method and apparatus employing at least one immobilized liquid membrane separated from a liquid sweep by a gas-permeable barrier for removing a gaseous component from a gaseous mixture. More particularly, the improvements include use of one or more passages extending through the gas-permeable barrier to effect transfer of active carrier species from the liquid sweep into the membrane, thereby maintaining the separation efficiency of each membrane.
Reference is made to copending application U.S. Ser. No. 806,845, filed June 15, 1977 (Walmet et al.) which generally relates to an improved method and apparatus for maintaining the separation efficiency of an immobilized liquid membrane (ILM) including introducing fresh liquid into a first region of the membrane to effect emission of at least partially deactivated liquid from a second region of the membrane. The reference application, which is assigned to the assignee hereof, is incorporated herein by reference.
A gas separation method and apparatus employing a feed gas mixture, an ILM, a liquid sweep, and a gas-permeable barrier disposed between the ILM and the liquid sweep are described in Ward application, U.S. Ser. No. 599,872, filed July 28, 1975, assigned to the assignee of this invention, incorporated hereby by reference, and hereinafter designated "Ward-A." The ILM in the Ward-A application contains in the liquid thereof at least one carrier species which is reversibly chemically reactive with the component to effect facilitated transport of the component through the membrane. The gas-permeable barrier specifically described therein is of a hydrophobic microporous material, which is substantially nonwettable by aqueous membrane liquids.
The method and apparatus of the Ward-A application are described therein with specific reference to removing hydrogen sulfide from a mixture of gases including carbon dioxide, e.g., coal gas. As described in Ward-A, a particularly suitable ILM for selectively removing hydrogen sulfide is an aqueous alkaline solution of potassium carbonate and potassium bicarbonate disposed in a microporous matrix layer of hydrophilic (i.e., wettable by the aqueous solution) composition.
Although the Ward-A method and apparatus are substantial advances in the art, it has been found that contact of coal gas, i.e., gasified coal, with the ILM results in deactivation of the membrane liquid. Such deactivation is believed to result from reaction of oxygen contained in the coal gas with hydrogen sulfide in the liquid membrane to form acidic sulfur oxides, such as sulfites, sulfates, and thiosulfates, which neutralize the alkaline carbonate/bicarbonate carrier species of the aqueous membrane liquid. Accordingly, there is a substantial need in the art for a method and apparatus by which the gas separation efficiency or selective permeability through a facilitated-transport ILM may be maintained.
As indicated in Ward-A, the method and apparatus described therein can be made even more selective to H.sub.2 S removal by adding gas barriers between adjacent ILM layers in the manner described in allowed copending Ward application U.S. Ser. No. 599,871, filed July 28, 1975, assigned to the assignee hereof and incorporated herein by reference (hereinafter "Ward-B"). That is, a membrane assembly including two ILM's having a gas-permeable barrier layer therebetween can be substituted for the single ILM in the Ward-A apparatus and method, thereby resulting in a sequential arrangement of an ILM/barrier/ILM/barrier. When multiple ILM's are employed in such an arrangement, it is more difficult to maintain the gas separation efficiency or selective permeability.
Additional background description of employment of ILM's for selectively separating hydrogen sulfide from a mixture of gases is provided by Ward et al. in U.S. Pat. No. 3,819,806, which is incorporated herein by reference.
It has now been found by practice of the present invention that substantial improvements for maintaining the separation efficiency of the above-described methods and apparatus are provided in simple efficient manner.